Grips are great if you've got big hands and you like a weighty camera. They're particularly useful for longer lenses as it helps the front to back balance of weight. Some also add power allowing for longer battery life.

They don't particularly protect the camera, although if you're going on and off a tripod a lot, it will cover that area. Basically it covers the bottom plate, so it depends on what you're doing.

I like having a battery grip on my camera because it gives me an extra shutter button for when I'm holding the camera vertically (so my right hand can be in a more comfortable position). On digital cameras you also get longer battery life. It's up to you if those things are worth the price.